Many applications where the fill level of a material in a container or a tank is measured require that the fill level be determined with a high degree of accuracy and/or precision. For example, in the pharmaceutical or food and beverage industries, accurate amounts of liquids need to be determined precisely for reaction processes. Accurate determinations of the amounts of material in containers are also required in the oil and gas industry, for example, where monetary transactions are based on the precise amount of material that is to change hands.
One of many methods for determining the fill level of a material in a container is through the use of radar based measurement devices. Radar based measurement devices come in many forms, but can generally be classified into two groups. These two groups are often referred to as “freely emitting” devices and “guided wave” devices.
A measurement device in the sense of the present invention is not to be understood as being restricted to a unitary collection of hardware components but can also be a system of spatially separated units. A measurement device can be viewed as comprising a transducer and a transmitter, wherein the transducer serves to convert a process variable, such as the fill level of a material in a tank, into an electrical signal, and wherein the transmitter serves to sample and process this electrical signal in order to produce a value for the process variable that corresponds to a physical situation that is to be measured. The transmitter, as the case may be, can further serve to transmit and/or save the determined process value for further use. The transducer generally comprises a microprocessor or microcontroller and various other electrical and electronic circuits. The transducer and the transmitter can be concentrated in a single unitary device, or they can be spatially separated. In the case where they are spatially separated, some sort of communication means, such as a cable or a wireless communication means, is provided. The distinction between transmitter and transducer can not always be strictly applied, as the transducer in some cases serves to preprocess a measurement signal and sometimes even comprises a microprocessor.
Guided wave radar measurement devices are used to measure the fill level of containers in applications where it is advantageous to concentrate the transmitted microwave energy around the waveguide. Signal losses can thereby be avoided, and power requirements can be reduced. The transmission signal for a guide wave radar device generally comprises electromagnetic pulses. The pulses are repetively produced at regular intervals of time. The time between pulses can be in the nanosecond range. After being produced, these pulses are coupled onto and guided along a wave guide, for example a cable or rod, in the direction the material that is to be measured. At the material interface, there is a sudden change in the dielectric constant—the change being proportional the difference between the dielectric constant of the material and the dielectric constant of a transmission medium that is between the radar device and the material, which is usually gaseous and, in particular, is usually air. A portion of the transmitted energy is reflected at this material interface due the change in impedance. This reflected portion of each pulse is then guided back to the radar device along the waveguide and sampled.
There are various means of sampling this pulsed signal. Commonly, a method is used in which the reflected pulses are mixed with a second pulsed signal that is generated in the measurement device. This secondary pulsed signal is produced with a repetition rate that differs slightly from the first pulsed signal so as to cause a stroboscopic effect that permits the received pulsed signal to be stretched in the time domain. This “stretched” signal can then be sampled with an analog to digital converter, wherein the converter is required to have a lower sampling rate than would normally be necessary in order to sample pulses in the nanosecond time range. The fill level measurement can be carried out based on time of flight methods. In principle, the time delay between the transmission and reception of a pulse corresponds to the distance between the radar measurement device and the material.
The magnitude of the change in the dielectric constant at the material interface plays a critical role in determining the signal strength of the reflected portion of each pulse. The signal strength decreases with a decrease in the magnitude of the change in the dielectric constant. In fill level measurement applications where the material to be measured has a small dielectric constant, a corresponding increase in the strength of the transmitted signal is necessary in order to insure that the reflected portion of the signal remains detectable. However, in order to increase the strength of the transmitted signal, the breadth of the pulses that are generated must be increased. This has the disadvantageous effect of decreasing the resolution with which the fill level measurement can be carried out. The resolution and/or precision of a fill level measurement of a guided wave radar measurement device is dependent on the sharpness of the pulses that are generated and transmitted.
In the US patent publication US 2013/0231877 A1 a method is disclosed for evaluating reflected measurement pulses of an electromagnetic signal that are transmitted along, for example, a coaxial probe arranged in a container. The method involves applying expectation values to received pulses in order to determine which pulse corresponds to the fill level in the event that there is an interference layer in the container. It is further disclosed that the method disclosed can be used on the spectrum of the intermediate frequency in an FMCW process instead of on the signal amplitude in the time range in order to thus measure the fill level of a material which is superimposed with at least one interference layer. The accuracy of the measurement is increased by reliably determining which pulse corresponds to the fill level of the material, and the precision of the measurement is increased by taking into account the effect of the interference layer on the time of flight of the electromagnetic signal.